Science of Sailing - Lesson 6

Topic: How a Sailboat Works: Sails

Teacher Resources:

Printable PDF Version of this Lesson Plan

Downloadable PPT of Lesson (5.3mb)

Lesson 6 Handout - Sail Area Exercise (PDF)

image from lesson 6 Click here to launch PowerPoint for Lesson 6

Primary Goal: After this lesson, students should be able to determine the final piece of their sailboat design, the sails. After reviewing how sails generate speed for a sailboat, they will learn how to generate speed for their boat while also taking into account the many other factors affected by a boat’s sail area.

Lesson Objectives:

• Students will review how sails are able to generate lift, and thus speed

• Students will learn the importance of and how to calculate Sail Area

• Students will learn how to calculate Sail Area – Displacement ratio

Lesson Outline:

  1. Intro
    1. A sailboat uses her sails for propulsion by generating lift (upwind) or by blocking the wind and being pushed (downwind)
    2. Just like a sports car is interested in a high horsepower – weight ratio, sailboats use a similar type of measurement to determine the potential speed, or acceleration, of the sailboat
  2. Sail Area
    1. A sailboat’s propulsion comes from the wind on her sails and is proportional to the area of all the sailboat’s sails
    2. This is measured by calculating the area of a each of the sails and then simply adding those numbers together
      1. The measurement of sail area is calculated using square feet
    3. For more advanced courses you can discuss the measurements of E and P for the mainsail and I and J for the headsail
      1. It is also worth noting that actual sail measurements are more complicated because of the curvature shape of the sails
  3. Sail Area – Displacement Ratio
    1. In order to compare sailboats with one another, we use the Sail Area – Displacement ratio
    2. This shows how much power the sails generate compared to each pound of displacement
      1. Under this calculation, we are assuming that displacement is the sole limit of a boat’s speed due to the reasons discussed in previous lessons
      2. The calculation also involves dividing the displacement by 64. This is done because the weight of seawater is 64 lb/ cubic foot
    3. The higher the ratio indicates a high performance sailboat usually designed for racing
    4. Because of the large sail area, these boats are sometimes more difficult to handle and can become easily overpowered in high winds
      1. Low ratio – 8-13
      2. Medium ratio – 14-20
      3. High ratio – 21-40+

Supplemental Resources:

Sailboat-Cruising.com: Sail Area Calculations

Cruising World: How Sailboats Measure Up


The actual calculation for Sail Area–Displacement is quite complicated. This is a great opportunity to give students practice using a scientific calculator. Also, the excel model provided can make quick calculations easier and can be used to check student’s work.

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